Dunes on the skeleton coast, Namibia (South West Africa): Geomorphology and grain size relationships

Abstract

AbstractSimple, and locally compound, transverse and barchanoid dunes dominate the 2000 km2 Skeleton Coast dunefield in northwestern Namibia/South West Africa. Dune height and spacing are closely correlated (r = 0‐89) and decrease across the dunefield from southwest to northeast, with an accompanying change from transverse to barchanoid ridges and ultimately barchans.The dunes are aligned transverse to the dominant strong south and south southwest onshore winds. Alignment patterns indicate that surface roughness changes between coastal plain and dunes cause dune‐forming winds to swing to the right over the dunes, but resume their original direction beyond.Grain size and sorting vary at three scales: the dune, the dune landscape and through the dunefield. Overall the sands, derived from three localities by deflation from beaches supplied by vigorous longshore drift, become progressively finer and better sorted across the dunefield paralleling changes in dune height and spacing.A statistically significant relationship (r = −0−65) was established between dune spacing and the phi grain size of the coarser fraction of the dune sands, demonstrating the importance of the protective effects of coarse grains, and suggesting that the morphometry of simple transverse dunes may be controlled by the scale of turbulence associated with the threshold wind speed required to move the coarsest fraction of the dune sand.